Optimization of magnet arrangement for nuclear magnetic resonance well logging tools

ABSTRACT

A method for fabricating a magnet on a nuclear magnetic resonance logging tool includes identifying a desired magnetic field strength at a predetermined depth of investigation, receiving a set of magnet segments configured for deployment on the logging tool in a magnet arrangement which make up a magnet array on the logging tool, processing a magnetic strength and an angular offset of selected magnet segments and their unique location in the magnet array to compute corresponding magnetic field strength solutions at the predetermined depth of investigation, selecting one magnet arrangement solution that minimizes a difference between the magnetic field strength solution and the desired magnetic field strength at the predetermined depth of investigation, and deploying the magnet segments on the logging tool according to the selected magnet arrangement solution to fabricate the magnet.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.15/087,675, entitled “Optimization of Magnet Arrangement for NuclearMagnetic Resonance Well Logging Tools”, filed Mar. 31, 2016 (U.S. PatentPublication No. 2017/0285120).

FIELD OF THE INVENTION

Aspects described relate to nuclear magnetic resonance well loggingtools. More specifically, aspects described relate to optimization ofmagnetic arrangements for nuclear magnetic resonance well logging toolsto reduce waste and increase economic production and output of sucharrangements.

BACKGROUND INFORMATION

Nuclear magnetic resonance well logging tools are vitally important tooil field service companies in efforts to obtain extraction ofhydrocarbons from geological stratum. Nuclear magnetic resonance welllogging tools use specially made arrangement magnets to investigate thegeological stratum to desired operating conditions. As a result,different well logging tools may be used if there are different desireddepths of investigation.

Nuclear magnetic resonance well logging tools are expensive to produce,however it is important to produce these well logging tools at aneconomical total cost. One of the most significant aspects of the costof nuclear magnetic resonance well logging tools is creation andarrangement of magnets in the tool. Reduction of the waste resultingfrom magnet creation provides significant cost savings.

Placement of magnets in a nuclear magnetic resonance well logging toolis conventionally performed using specially designed magnets. Thesemagnets, however, are not optimal in their overall performance. As aresult, static magnetic field (B₀) produced by conventional, welllogging tools deviates from the original design. This imperfect magneticfield distribution may produce a distorted nuclear magnetic resonancesignature during use of the well logging tool.

In an attempt to produce a more consistent static magnetic field,manufacturers attempt to control the overall static magnetic field byadding shims to the magnetic material package. These shims, coupled withremoval of magnetic material, for example, can influence the overallstatic magnetic field delivered by the nuclear magnetic resonance tool.Problems with this type of arrangement abound. Careful mapping anddetermination of the strength of magnets being installed in the toolmust be accomplished in order to determine the field strength. Placementof shims inside a tool, additionally, takes up vital space in the insideof the tool that could be used for more practical purposes.

SUMMARY

This summary is provided to introduce a selection of concepts that arefurther described below in the detailed description. This summary is notintended to identify key or essential features of the claimed subjectmatter, nor is it intended to be used as an aid in limiting the scope ofthe claimed subject matter. A method for fabricating a magnet on anuclear magnetic resonance logging tool is disclosed. The methodincludes identifying a desired magnetic field strength at apredetermined depth of investigation about the logging tool. A set ofmagnetic segments are received, the magnetic segments being configuredfor deployment on the logging tool in a magnet arrangement comprising aplurality of the magnet segments which make up a magnet array on thelogging tool, each of the magnet segments having a correspondingmagnetic strength and angular offset. A plurality of magnet arrangementsolutions is obtained in which selected ones of the magnet segments arearranged in the magnet array on the logging tool wherein each selectedmagnet segment is deployed in a unique location on the logging tool. Themagnetic strength and angular offset of each selected magnet segment andthe unique location of each selected magnet segment arranged in themagnet array of each of the obtained magnet arrangement solutions isprocessed to compute corresponding magnetic field strength solutions atthe predetermined depth of investigation for each of said plurality ofmagnet arrangement solutions. One of said magnet arrangement solutionsis selected that minimizes a difference between the magnetic fieldstrength solution and the desired magnetic field strength at thepredetermined depth of investigation. The selected magnet segments aredeployed on the logging tool according to said selected magnetarrangement solution to fabricate the magnet.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee. In the drawings, sizes, shapes, and relativepositions of elements are not drawn to scale. For example, the shapes ofvarious elements and angles are not drawn to scale, and some of theseelements may have been arbitrarily enlarged and positioned to improvedrawing legibility.

FIG. 1 is a graph of a relationship between excitation pulse frequencyand depth of investigation (in length units).

FIG. 2 is a side view of a construction of a cylindrical magnet array ofa set of magnet segments, wherein each segment has a nominal directionparallel to the cylinder axis.

FIG. 3 is a distribution graph of remnant magnetization for 387 magnetsegments.

FIG. 4 is a comparison graph of a tool with a uniform static magneticfield vs. a non-uniform static magnetic field.

FIG. 5 is a graph of cylindrical magnet array properties whenconstructed from a set of magnet segments with specific angular offsetdistributions.

FIG. 6 is a graph of optimized segment arrangements for seven innerrings of cylindrical arrays.

FIG. 7A is example of segment distribution of magnets to be used fordownhole tools.

FIG. 7B is a resulting static magnetic field B₀ at a point of interestfrom the segment distribution of FIG. 7A.

DETAILED DESCRIPTION

An aspect of this disclosure is to reduce the wasted magnet material(hence magnet cost) to maintain the consistency in fabricating magnetsfor Nuclear Magnetic Resonance (NMR) tools. This aspect is realized byoptimizing the arrangement of non-perfect magnet segments, so that thecombination of the segments produces sufficiently homogeneous magneticfields that meets various NMR requirements. This is accomplished withoutusing additional shimming materials that use limited space on an NMRsensor.

The sensor of the Nuclear Magnetic Resonance (NMR) well logging toolsconsists of permanent magnet(s) and antenna(e) to project a staticmagnetic field (B₀) and oscillating magnetic field (B₁) into a formationsurrounding a wellbore. A resulting tool sensitive region is determinedby a resonance frequency of nuclear spins, which is proportional to thelocal static magnetic field B₀ strength, and the frequency bandwidth ofan excitation signal, which is proportional to local oscillatingmagnetic field B₁ strength as described in FIG. 1.

As provided in FIG. 1, a relationship between the static magnetic field,B₀, oscillating magnetic field, B₁, and the sensitive region, asdescribed above, is illustrated. The resonance frequency of nuclearmagnetization, f₀, is proportional to local static magnetic fieldstrength B₀ through a coefficient γ. For an excitation pulse offrequency f_(rf), the depth of investigation (DOI) is determined by thelocation where the equation f_(rf)=f₀=γB₀ is satisfied. In addition,thickness of the sensitive region is determined by the homogeneity ofthe static magnetic field strength B₀ and the bandwidth of theexcitation pulse, which is proportional to the pulsed magnetic fieldstrength B₁.

If the nominal static magnetic field strength B₀ deviates from a targetvalue, then the depth of investigation (DOI) will shift closer to orfurther from the tool, resulting in potential signals coming fromunwanted regions (e.g., the borehole). If static magnetic field strengthB₀ homogeneity is compromised, then the resonance frequency also spreadsacross the region of interest, and only a fraction of the regions willbe excited by the oscillating magnetic field strength B₁ of a givenbandwidth. The result is a reduced NMR signal. Nominal static magneticfield strength B₀ and the homogeneity of the static magnetic fieldstrength B₀ around the region of interest are one of the most importantproperties of an NMR sensor.

Each NMR well logging tool has a unique operating profile and associatedrequirements for the sensitive region. For example, wireline tools oftenrequire an elongated sensitive region along a tool axis to scan theformation at relatively high speed (up to 3,600 ft/hour). Logging WhileDrilling (LWD) tools, meanwhile, require an axisymmetric sensitiveregion to conduct NMR measurements under rotating conditions. Effortshave been made to generate specific static magnetic field strength B₀and oscillating magnetic field strength B₁ distributions to satisfy theabove requirements.

To realize specific static magnetic field strength B₀ distribution, themagnet is usually composed of multiple (from 10s to 100s) segments, eachof which is magnetized in a pre-defined direction. The tolerances ofthose segments, however, are limited by the complicated manufacturingprocess. First, a mixture of rare earth magnetic materials, such asSamarium (Sm) and Cobalt (Co), and other ingredients are melted in afurnace to obtain a cast ingot. The ingot is then crushed, pulverized,and milled into small particles of several micrometers. The obtainedalloy particles are compressed in a die to be formed into magnet blockswhile aligning the orientations with applied magnetic field. The magnetblock is then sintered, heat treated, magnetized, and shaped. If shapingneeds to happen after magnetization, a special technique, such asElectric Discharge Machining (EDM), shall be used to avoid mechanicaldamage.

The above process introduces variations both in the strength andmagnetizing direction. As a result, an ensemble of magnet segmentsexhibits certain distribution, which varies from batch to batch. FIG. 2shows an example of a cylindrical magnet array that consists of 168segments. Each segment has a nominal magnetizing direction parallel tothe cylinder axis. FIG. 3 shows the remnant magnetization (B_(r)) of 387segments prepared to build a magnet array. The material (SmCo R32H) isdesigned to have B_(r)=1.12˜1.20 T, but the actual distribution isshifted towards the lower end in this particular example. If thesesegments are randomly arranged on a cylinder, the resulting staticmagnetic field strength B₀ will be distorted and lose axisymmetricproperties as provided in FIG. 4. This will cause inconsistent signalamplitude in LWD operation. Furthermore, deviation of the staticmagnetic field strength B₀ causes the shift of the DOI from the targetlocation.

Referring to FIG. 2, the construction of a cylindrical magnet array isillustrated wherein the array consists of a set of magnet segments. Eachsegment has a nominal magnetizing direction parallel to the cylinderaxis as indicated by the arrow.

Referring to FIG. 3, a distribution of remnant magnetization (B_(r)) for387 magnet segments is illustrated. The material (SmCo R32H) is supposedto have B_(r)=1.12˜1.20 T.

Referring to FIG. 4, a shape of the sensitive region dictated by staticmagnetic field strength B₀ distribution is illustrated. If the staticmagnetic field strength B₀ is uniform around the tool, then the DOI willbe uniform as well as illustrated on the left. If the static magneticfield strength B₀ is not uniform, then the DOI is distorted and the NMRsignal will be compromised in the rotating operation as illustrated onthe right. This requires static magnetic field strength B₀ consistencywithin a tool and among multiple tools.

To avoid such tool performance degradation, stringent pass/fail criteriaare imposed on individual segments assuming the worst case scenario(e.g., all segments are tilted in the same way). For example, FIG. 5shows the likelihood of magnet properties when segments of certaintolerance are randomly distributed on a cylinder. For this particularexample, the angular offset is within two (2) degrees to satisfy toolrequirements (i.e., 47.5±0.5 G for field strength to maintain theconsistency of DOI, and 0.2 G for Angular Variation to maintain theuniformity of cylindrical sensitive region). Segments beyond +/−2 degreetolerance must be discarded. This severely brings down the yield rate ofthe manufacturing process and thus is reflected in the high cost ofmagnet arrays.

Referring to FIG. 5, a likelihood of cylindrical magnet array propertieswhen constructed from a set of magnet segments with specific angularoffsets is illustrated. Each distribution represents the histogram of5,000 magnet array implemented randomly.

One aspect described herein is to reduce the wasted magnet material(hence magnet cost) by optimizing the arrangement of non-perfect magnetsegments, so that the combination of the segments produces sufficientlyhomogeneous static magnetic field strength B₀ that meets various NMRrequirements. This shall be done without using additional shimmingmaterials that eat up limited space on an NMR sensor.

The optimization involves the evaluation of a magnetic field produced bya set of magnet segments. When the point of interest is at a largedistance from the source dipoles, the magnetic field may be approximatedby a dipole field. The magnetic field flux density at point r is givenby the below equation:

$B = {\frac{\mu_{o}}{4\;\pi}\left\lbrack {\frac{3{r\left( {m \cdot r} \right)}}{R^{5}} - \frac{m}{R^{3}}} \right\rbrack}$where m is magnetic moment, R is the distance between the source m andpoint r, and μ₀ is the permeability of free space. The total field maybe obtained by integrating the contribution of magnetic dipoles thatrepresent each segment. Although such a simple dipole model is fast andconvenient in optimization process that involves many iterations, FiniteElement Analysis (FEA) or any other methods may also be used tocalculate the magnetic field.

The magnetic dipole moment is a vector that has the magnitude anddirection, both of which show variations in real-world magnets. The goalof this optimization is to find a set of m, within which small localvariations cancel out with each other. Then the resulting staticmagnetic field B will have a global distribution that is sufficientlyuniform over the region of interest.

A set of magnet segments may be sorted out based on the quality prior tothe arrangement optimization. The example of quality includes themagnetization strength and the angular offset. The closer themagnetization is to the pre-determined optimal value, the better themagnet. Also, the smaller the offset angle is, the better the magnet.Once the magnets are sorted, the magnets are evenly split betweenmultiple magnet assemblies, so that each assembly has a distribution ofgood and poor segments.

The optimization may be implemented to minimize the cost function thatrepresents the quality of the magnet assembly. One example of magnetquality is the deviation from the target field value. By using multiplepoints, field homogeneity may be defined in the region of interest. Forexample, for a cylindrical magnet given in FIG. 2, the variations ofmagnetic field in both the axial and angular directions may be minimizedin a single run of optimization. Other types of magnet array may havesensitive regions of different shapes, but similar criteria may bedefined for optimization.

There are multiple algorithms to realize the optimization. The easiestbut most inefficient way is the Monte-Carlo method, where thecombinations of magnet segments are randomly chosen to compare theperformance of resulting magnet array. For example, for the cylindricalmagnet given in FIG. 2 made out of magnet segments given in FIG. 3, thesmallest angular variation was found to be 0.0085 G after 10,000iterations, which is one order of magnitude better than the averagevalue 0.053 G obtained in the same iterations. The drawback of thisapproach is the coverage of the solution space; in the above example,there are 387P168 candidate solutions (i.e., select 168 segments out of387 segments and assemble them in a particular order) to be evaluated,which is a large number and will, for most purposes, not be thoroughlyinvestigated. This would result in sub-optimal results. For the purposeof the disclosed aspects, sub-optimal solutions are acceptable as longas the static magnetic field strength B₀ variations are small enoughcompared to the errors introduced by other factors (e.g., mechanicaltolerance in locating magnet segments on a cylindrical collar).

To obtain similar or even better results, a more efficient method is toevolve candidate solutions based on the result of the previousiteration. This will allow the finding of (quasi-)optimal solution(s)without evaluating all the candidates. The examples of optimizationalgorithms suitable for combinational optimization problem like thisincludes, but not limited to, Genetic Algorithm, Simulated Annealingmethod, Taboo Search method, Simulated Evolution method, StochasticEvolution method, and Hybrid method (i.e., a combination of them), amongothers.

Regardless of the algorithms being used, the optimization process willensure segment configuration that generates a static magnetic field B₀with the smallest deviations from the target distribution. For example,FIG. 6 shows the optimized segment arrangement for the inner 7 rings ofcylindrical magnet arrays. The horizontal axis is the segment positionon a collar specified in angle, and the vertical axis is the segmentstrength. Designated lines represent the result for a pair of magnetarrays facing with each other. It can be seen that, at some point, astrong segment in inner ring is compensated by a weaker segment in outerring. Also, good segments were located on the inner most ring, while thebad segments were left away in the outer rings (i.e., far from thesensitive region).

Referring to FIG. 6., an optimized segment arrangement for the inner 7rings of cylindrical magnet arrays from FIG. 2 is displayed. Theoptimization process will typically place the segments of good qualityclose to the sensitive region, while the process will move ones withnon-optimal quality to the far end of the spectrum. The optimizationprocess will typically place a segment with positive variation next tothe one with negative variation, so that the local variations cancel outwhen the field point is at a large distance.

Referring to FIG. 7, a resulting static magnetic field B₀ isillustrated. The noted lines represent the distribution of segmentproperties with loose and tightened tolerances. If 100 magnets are builtfrom a set of segments specified, the resulting magnetic field can behigher or lower than the target value, and there is a significant chanceto receive a magnet that is out of the spec. If optimization isimplemented, the same distribution delivers magnets, which areconsistently around the target value, 47.5 G. In other words, theoptimization saves the bad segments that are otherwise discarded.Considering the high value of rare-earth magnet materials, the reductionof wasted materials has a significant impact on the magnet cost.

As demonstrated above, the optimization process provides consistentstatic magnetic field B₀ out of a sub-optimal set of magnet segments.This will contribute to maintain the DOI for fixed operating frequency.The same technique may be used to reduce the variation of operatingfrequency (hence reduce the need for antenna/electronics adjustment) forthe fixed target DOI.

Referring to FIGS. 7A and 7B an example of segment distribution (top)and resulting B₀ field at a point of interest (bottom) are illustrated.In the top figure, the blue and red lines represent the segmentproperties with loose and tightened tolerances. If 100 magnets are builtfrom such segments, the resulting static magnetic field can be higher orlower than the target value, and there is a significant chance toreceive a magnet that is out of the spec (bottom). However, if theoptimization is implemented, the same blue distribution delivers thegreen line, which is consistently around the target value, 47.5 G. Theoptimization will save the bad segments that are otherwise discarded.

In one example embodiment, a method to produce a magnet arrangement,comprising selecting a depth of investigation to be achieved by adownhole tool, identifying a desired magnetic field strength at thedepth of investigation, producing a set of magnets to be incorporatedinto the downhole tool, sorting the set of magnets based on a quality ofeach of the magnets and optimizing the set of magnets such that thequality of each of the magnets results, when arranged, in the desiredmagnetic field strength at the depth of investigation and wherein theoptimizing minimizes a cost function of the set of magnets produced.

In a further example embodiment, the method may be accomplished whereinthe identifying the desired magnetic field strength at the depth ofinvestigation is approximated by a dipole field.

In a further example embodiment, the method may be accomplished whereina magnetic flux density at a point r for the depth of investigation iscalculated as:

$B = {\frac{\mu_{o}}{4\;\pi}\left\lbrack {\frac{3{r\left( {m \cdot r} \right)}}{R^{5}} - \frac{m}{R^{3}}} \right\rbrack}$where m is a magnetic moment, R is the distance between a source m andpoint r, and μ₀ is the permeability of free space.

In a further example embodiment, the method may be accomplished whereinthe quality is a magnetization strength.

In a further example embodiment, the method may be accomplished whereinthe quality is an angular offset.

In a further example embodiment, the method may be accomplished whereinthe quality is both a magnetization strength and an angular offset.

In a further example embodiment, the method may further comprisingsplitting the set of magnets between multiple magnet assemblies beingconstructed.

In a further example embodiment, the method may be accomplished whereinthe optimizing the set of magnets is performed through a Monte Carlomethod.

In a further example embodiment, the method may be accomplished whereinthe optimizing the set of magnets is performed through at least one of agenetic algorithm, a simulated annealing method, a taboo search method,a simulated evolution method and a stochastic evolution method.

A few example embodiments have been described in detail above; however,those skilled in the art will readily appreciate that many modificationsare possible in the example embodiments without materially departingfrom the scope of the present disclosure or the appended claims.Accordingly, such modifications are intended to be included in the scopeof this disclosure. Likewise, while the disclosure herein contains manyspecifics, these specifics should not be construed as limiting the scopeof the disclosure or of any of the appended claims, but merely asproviding information pertinent to one or more specific embodiments thatmay fall within the scope of the disclosure and the appended claims. Anydescribed features from the various embodiments disclosed may beemployed in combination. In addition, other embodiments of the presentdisclosure may also be devised which lie within the scope of thedisclosure and the appended claims. Additions, deletions andmodifications to the embodiments that fall within the meaning and scopesof the claims are to be embraced by the claims.

Certain embodiments and features may have been described using a set ofnumerical upper limits and a set of numerical lower limits. It should beappreciated that ranges including the combination of any two values,e.g., the combination of any lower value with any upper value, thecombination of any two lower values, or the combination of any two uppervalues are contemplated. Certain lower limits, upper limits and rangesmay appear in one or more claims below. Numerical values are “about” or“approximately” the indicated value, and take into account experimentalerror, tolerances in manufacturing or operational processes, and othervariations that would be expected by a person having ordinary skill inthe art.

The various embodiments described above can be combined to providefurther embodiments. These and other changes can be made to theembodiments in light of the above-detailed description. In general, inthe following claims, the terms used should not be construed to limitthe claims to the specific embodiments disclosed in the specificationand the claims, but should be construed to include other possibleembodiments along with the full scope of equivalents to which suchclaims are entitled. Accordingly, the claims are not limited by thedisclosure.

What is claimed is:
 1. A method for fabricating a magnet on a nuclearmagnetic resonance logging tool, the method comprising: identifying adesired magnetic field strength at a predetermined depth ofinvestigation about the logging tool; receiving a set of magnet segmentsconfigured for deployment on the logging tool in a magnet arrangementcomprising a plurality of the magnet segments which make up a magnetarray on the logging tool, each of the magnet segments having acorresponding magnetic strength and angular offset; obtaining aplurality of magnet arrangement solutions of selected ones of the magnetsegments arranged in the magnet array on the logging tool wherein eachselected magnet segment is deployed in a unique location on the loggingtool; processing the magnetic strength and angular offset of eachselected magnet segment and the unique location of each selected magnetsegment arranged in the magnet array of each of the obtained magnetarrangement solutions to compute corresponding magnetic field strengthsolutions at the predetermined depth of investigation for each of saidplurality of magnet arrangement solutions; selecting one of said magnetarrangement solutions that minimizes a difference between the magneticfield strength solution and the desired magnetic field strength at thepredetermined depth of investigation; and deploying the selected magnetsegments on the logging tool according to said selected magnetarrangement solution to fabricate the magnet.
 2. The method according toclaim 1, wherein said processing comprises computing the magnetic fieldstrength solutions by integrating the contribution of magnetic dipolesthat represent each of the magnet segments.
 3. The method according toclaim 2, wherein a magnetic flux density at a point r for the depth ofinvestigation is calculated for each of the magnet segments as:$B = {\frac{\mu_{o}}{4\;\pi}\left\lbrack {\frac{3{r\left( {m \cdot r} \right)}}{R^{5}} - \frac{m}{R^{3}}} \right\rbrack}$where m is a magnetic moment, R is a distance between a source magneticmoment m and the point r, and μ₀ is the permeability of free space. 4.The method according to claim 1, wherein said plurality of magnetarrangement solutions are obtained by randomly choosing using a MonteCarlo method.
 5. The method according to claim 1, wherein said pluralityof magnet arrangement solutions are obtained by using at least one of agenetic algorithm, a simulated annealing method, a taboo search method,a simulated evolution method, or a stochastic evolution method.
 6. Themethod of claim 1, wherein: said processing further comprises processingthe magnetic strength and angular offset of each selected magnet segmentand the unique location of each selected magnet segment to computecorresponding angular variations of the magnetic field strengthsolutions at the predetermined depth of investigation for each of saidplurality of magnet arrangement solutions; and said selecting comprisesselecting said magnet arrangement solution that further minimizes theangular variation in the magnetic field strength solution at the depthof investigation.
 7. The method of claim 1, wherein: said processingfurther comprises processing the magnetic strength and angular offset ofeach selected magnet segment and the unique location of each selectedmagnet segment to compute corresponding axial variations of the magneticfield strength solutions at the predetermined depth of investigation foreach of said plurality of magnet arrangement solutions; and saidselecting comprises selecting said magnet arrangement solution thatfurther minimizes the axial variation in the magnetic field strengthsolution at the depth of investigation.
 8. The method of claim 1,wherein said receiving the set of magnet segments comprises (i)producing the set of magnet segments and (ii) obtaining the magneticstrength and angular offset of each of the magnet segments.
 9. Themethod of claim 1, wherein obtaining the plurality of magnet arrangementsolutions comprises: sorting the set of magnet segments into goodsegments and poor segments based on said magnetic strengths and angularoffsets, wherein the good segments include the magnet segments that meetpredetermined magnetic strength and angular offset criteria and the poorsegments include the magnet segments that fail to meet the predeterminedmagnetic strength and angular offset criteria; and obtaining theplurality of magnet arrangement solutions such that each magnetarrangement solution includes a distribution of the good segments andthe poor segments.
 10. The method of claim 9, wherein the magnetarrangement comprises a plurality of rings of the magnet segments whichmake up a cylindrical magnet array on the logging tool.
 11. The methodof claim 10, wherein the plurality of rings of the magnet segments ofthe cylindrical magnet array on the logging tool include inner rings andouter rings, wherein the good segments are located in the inner ringsand the bad segments are located in the outer rings.
 12. The method ofclaim 1, wherein the magnet array on the logging tool is a cylindricalmagnet array on the logging tool.
 13. The method of claim 12, whereinthe magnet arrangement comprises a plurality of rings of the magnetsegments which make up the cylindrical magnet array on the logging tool.